core.sh: drop GNU stat (coreutils) dependency

Do not use 'stat' anymore because switches are different from BSD version.
Credits go to thomas.tan
Fixes issue 945.
multiup.org: initial module with probe & list

Fixes issue 938
plowup: update $ERR_LINK_NEED_PERMISSIONS error message
core.sh (check_exec): better version (use command builtin)